PHP's popularity and simplicity made it easy for the company's developers to quickly build new features. But PHP's (lack of) performance makes scaling Facebook's site to handle hundreds of billions of page views a month problematic, so Facebook has made big investments in making it leaner and faster. The latest product of those efforts is the HipHop VM (HHVM), a PHP virtual machine that significantly boosts performance of dynamic pages . And Facebook is sharing it with the world as open-source.

I'm too lazy at the moment to track down all of the reasons why using a "proper RDBMS" is a bad idea for facebook but that gives you some hints anyways: licensing costs, speed of support issue resolution and lack of proof that it could actually handle the load.

proper RDBMS == true scotsman. By the time you got something that worked as well as what they use now, it would be almost exactly what they use now, but cost a but load more and have an "oracle" sticker on it.

MySQL and NoSQL databases aren't really much better than running say Postgres with certain options turned off which IMHO is a decent ORDMS and is in the same league as Oracle and MS-SQL and whatever IBM push (DB2?).

TBH I am not a hardcore database guy but I know enough tricks to optimize queries when needed. But when I am using MySQL you just get that feeling it is a bit shitty.